This invention relates to a process for the chemical-mechanical treatment and purification of ground waters, surface waters and effluents of various types employing reagents involving coagulation, flocculation and precipitation and agents conducive to sedimentation and altering of the pH value.
As is well known, various types of flocculation processes have been employed for a long time in the treatment and purification of waters and effluents. Experience indicates that, especially in the case of strongly polluted waters, adequate purification effects can only be obtained if a part of the so-called flocculated sludge is recirculated. In the most common version of such suspension recirculation processes, the so-called sludge contact process, the raw water mixed with flocculants and/or precipitants and/or flocculant aids and/or agents conducive to flocculation and, if applicable, chemicals altering the pH value, comes into contact with the so-called contact sludge brought into circulation many times.
The contact sludge is continuously separated from the so-called clarified water in a sedimentation zone which is generally arranged in the form of a circle around a central flocculation area and, subsequently, admixed to the raw water in a reaction zone.
The sludge contact process can, in principle, be subdivided into the following five process phases:
Phase I: First flocculation phase (first initial process stage)--Admixture of one or several flocculants (for example iron or aluminum compounds or salts) flocculant aids for example synthetic nonionogenic or anionic or cationic products or activated silica) and/or one or several precipitants (for example calcium hydroxide, calcium oxide or calcium salts) and/or one or several agents conducive to sedimentation (for example ground limestone) and/or if necessary, one or several reagents for altering the pH value. PA1 Phase II. Sludge contact phase (second initial process stage)--Admixture of contact sludges by means of stirring and, if applicable, the further admixture of agents of the type indicated under phase I. PA1 Phase III. Second flocculation phase (third initial process stage)--Admixture of so-called flocculant aids (for example synthetic cationic, nonionogenic or anionic products on polyacrylamide basis and/or natural products or transformed natural products on starch or alginate basis and/or inorganic products, such as activated silica, bentonites, and the like) by means of stirring, and if applicable, admixture of agents of the type indicated under Phase I, utilizing an absorbed stirring energy of more than 20 Watts per m.sup.3 of the reaction volume available in this zone, and limiting the retention time to 5 minutes at the most. PA1 Phase IV: Phase for the formation of coarse flocs (fourth initial process stage)--Completion of reaction of the previously added reagents and agents and formation of settleable particles and settleable conglomerations of particles (flocs and reaction products and dispersed components of raw water) together with absorbed and attached and occluded raw water components with at most moderate movement. PA1 Phase V: Separation phase (further process stage)--Separation of flocs and reaction products and raw water components from the clarified water (pure water) and separate removal of the separated solids (sludge) and of the clarified water.
In the case of known sludge contact clarification plants the process phases, as a rule, do not generally run in reaction areas or reaction zones which are adequately separated from each other and strictly allocated to the individual process phases, but, several process phases are always combined. The tailoring of optimum conditions for each of the process phases is not possible by such a process procedure. Due to the low solids concentration of the return sludge, a multiple recirculation of the water to be treated is necessary; moreover, the volume of the water quantity to be circulated is commonly three to ten times that of the plant throughput. This brings the result that in spite of the addition of large quantities of reagents the residual content of suspended solids, remaining in the clarified water and mostly finely dispersed, remains relatively large and, in addition, considerable quantities of flocculant aids added are drawn off, unused, with the clarified water.
Due to the long total reaction time and settling time required, the large size of the plants proves to be a particular disadvantage. The resulting idle times make even the manual regulation of the dosing quantity of the agents employed for treatment purposes, difficult to a high degree. Automatic process control, in particular in the case of strongly fluctuating raw water characteristics, is an illusion.
The filters, frequently located after the sludge contact clarifiers for purposes of further treatment of the water, must be filled with very fine filter material with thick filter layers since the retention of, in particular, the finely dispersed particles would otherwise not take place or to only a limited extent.
In addition to the high additional investment costs caused by this, considerable maintenance expenditure is necessary due to the shorter filter life, resulting therefrom. The results which are thus obtainable are generally unsatisfactory and often insufficient, when measured, in particular, against the requirements placed in recent years for potable water and service water quality, which have increased appreciably compared to previous standards.